I am running D1-MIUI-0.10.15-v2-MOD and tried to download the new YouTube app Google pushed out today (v2.0.26) and of course got "Installation Unsuccessful". If anyone is interested, here's what I did to fix it:
First I backed up the YouTube app that came with then OS, then in ADB I entered the following commands:
mount -o remount,rw -t yaffs2 dev/block/mtdblock4 system
busybox rm system/app/YouTube.apk
mount -o ro,remount -t yaffs2 dev/block/mtdblock4 system
I then rebooted my phone and was able to successfully download and install the new YouTube app from Market... if anyone has any questions or issues, feel free to ask!